Leads and their associated electrodes or electrode tips originally used with asynchronous heart pacemakers were primarily designed to provide a low pacing threshold for continuous pulses that are generated at a predetermined rate regardless of any natural or unnatural heart activity. The next generation of cardiac pacemakers began with development of the demand pacemaker. With this type of pacemaker, the electrode served a dual purpose. It not only provided the pacing pulse to the heart, it was also used to electrically sense intrinsic heart activity for the purpose of suppressing a pacing pulse whenever a natural heartbeat occurred within a predetermined time period after the last natural heartbeat or pacing pulse. Similarly, development of the synchronous pacemaker required a pacing pulse to be provided either in the absence of a natural heartbeat, or in synchronism with the occurrence of a natural heartbeat whenever it occurred within a predetermined time period after the last pacing pulse. The electrode utilized with a synchronous pacemaker also served a dual function of providing a pacing pulse and electrically sensing the natural heartbeat. Subsequent heart pacemakers have become increasingly complex, and have numerous additional modes of operation which are selectable by a doctor according to a patient's physiologic needs. However, these increasingly complex pacemakers continue to utilize electrodes which serve the dual function of providing the pacing pulse and also sensing heart activity.
The requirements for an optimum pacing electrode and an optimum sensing electrode are sometimes conflicting. For example, an optimum pacing electrode would have a very small surface area exposed to the heart tissue in order to achieve a low pacing threshold and a high current density. However, a sensing electrode requires a large surface area in order to detect a relatively low level electrical signal indicating natural heartbeat activity. Prior art electrodes have attempted to balance these two requirements by sensing and pacing through the same exposed electrode surface, thus compromising the requirement of a small surface area for attaining a low pacing threshold and a large surface for detecting relatively weak electrical signals. Porous tip electrodes utilizing a platinum mesh have been developed such as that disclosed in U.S. Pat. No. 4,156,429. However, this type of prior art electrode utilizes a filament compress in which the filaments form between 3% and 30% of the total volume of the fibrous portion. However, this type of electrode still utilizes the same electrode surfaces for both pacing and sensing. The improved electrode according to one embodiment of the invention eliminates these problems by providing a large sensing area, a small pacing area, and a porous electrode tip which provides passive fixation to the endocardium.